Conventionally, a power amplifier (PA) for amplifying power of a transmission radio signal used in a wireless communication device consumes a large amount of power among the building elements of the wireless communication device. Improving the PA power efficiency is therefore considered to be important.
In recent wireless communication, amplitude modulation is dominant for a higher spectral efficiency of radio signals for use. A typical example is QAM (Quadrature Amplitude Modulation) which is a modulation scheme in digital wireless communication.
In this modulation scheme, the PA requires high linearity with less distortion to prevent degradation of a transmission signal and the influence on an adjacent channel. Conventionally, a PA used in an amplitude modulation wireless communication device operates in, a large backoff (low input power) state which improves the linearity, achieving a low distortion characteristic.
However, when the PA operates in a large backoff state, that is, in a power region lower than the saturation power of the PA, the PA power efficiency decreases.
To solve a challenge to the PA characteristic to achieve both high power efficiency and linearity, “polar modulation” have extensively been studied as a technique for improving the power efficiency in a large backoff (low input power) state. PAs having satisfactory linearity and high power efficiency using polar modulation have been developed.
FIG. 11 is a block diagram showing an outline of a power amplifier based on an ET (Envelope Tracking) scheme which is a kind of conventional polar modulation. FIG. 12 is a block diagram showing an outline of a power amplifier based on an EER (Envelope Elimination and Restoration) scheme. The arrangement and operation of the power amplifier using the polar modulation technique will be explained.
An ET power amplifier 10 shown in FIG. 11 includes a polar modulator 11, power supply modulator 12, and RF (Radio Frequency) amplifier 13. The power amplifier 10 implements power amplification of an input transmission signal.
Upon receiving transmission signal data at an input terminal 11-a, the polar modulator 11 outputs an amplitude component signal A of the transmission signal to an output terminal 11-b and, to an output terminal 11-c, an RF modulated signal C obtained by superposing the amplitude component and phase component of the transmission signal data on a carrier. The polar modulator 11 has even a function capable of setting the output timings of the amplitude component signal A and RF modulated signal C to desired values individually.
The power supply modulator 12 amplifies the amplitude component signal A to generate an amplified amplitude component signal B, and outputs the amplified amplitude component signal B to a power supply terminal 13-a of the RF amplifier 13.
The RF amplifier 13 receives the RF modulated signal C output to the output terminal 11-c of the polar modulator 11, and modulates the amplified amplitude component signal B input to the power supply terminal 13-a of the RF amplifier 13. A transmission signal D, which is an RF modulated signal obtained by superposing the amplitude component and phase component of the transmission signal data on a carrier and amplifying the resultant signal, is output to an output terminal 13-b of the RF amplifier 13.
The ET power amplifier 10 controls a voltage signal to be input to the power supply terminal 13-a of the RF amplifier 13 in accordance with the amplitude of the RF modulated signal C. More specifically, the power amplifier 10 decreases the voltage of a voltage signal to be input to the RF amplifier 13 when the power of the RF modulated signal C is low, and increases it when the power of the RF modulated signal is high. Hence, power wastefully consumed when an output from the RF amplifier 13 is low can be suppressed, improving the power efficiency.
An EER power amplifier 20 shown in FIG. 12 having the same arrangement as that of the ET power amplifier 10 adopts a technique of outputting, to an output terminal 21-c of a polar modulator 21, an RF phase modulated signal E obtained by superposing the phase component of a transmission signal on a carrier, inputting the RF phase modulated signal E to the RF amplifier 13, and amplifying the transmission signal.
Also in this case, a signal at the power supply terminal 13-a of the RF amplifier 13 is modulated in accordance with the amplitude component signal B. More specifically, the power amplifier 20 decreases the power supply voltage of the RF amplifier 13 when the amplitude of the transmission signal is small (low output), and increases it when the amplitude of the transmission signal is large (high output). Power wastefully consumed by a conventional power amplifier when an output from the RF amplifier is low can be suppressed, improving the power efficiency.
However, the power amplifiers 10 and 20 using these polar modulation schemes request the power supply modulator 12 of all wide band (high speed), wide dynamic range (high voltage and low noise), and high power efficiency characteristics. It is hard for the conventional techniques to meet these requests. More specifically, a transistor which forms the power supply modulator 12 tends to decrease the operation speed as the breakdown voltage rises. It is generally difficult to achieve both a high voltage operation and wide band (high speed) characteristic. A power amplifier which operates with relatively high power consumption in a wireless communication device hardly satisfies both a wide dynamic range and wide band (high speed characteristic).
For example, when the power supply modulator 12 is implemented by a linear regulator, it can achieve both a wide band (high speed) characteristic and wide dynamic range (low noise) characteristic. However, it is difficult to achieve high power efficiency due to dissipation to heat. When the power supply modulator 12 is implemented by a switching regulator, it can achieve high power efficiency, but can achieve neither a wide band (high speed) characteristic nor wide dynamic range (low noise) characteristic.
To solve these problems of the power amplifier using the polar modulation scheme, there has been proposed a technique (patent literature 1) in which an error correction unit for correcting a signal error is arranged in a power supply modulator formed from a high-power-efficiency switching amplifier, thereby achieving both a high power efficiency characteristic and low noise characteristic.
Power amplifiers which achieve concurrently a high power efficiency, a wide band, and low noise characteristic are also proposed by a technique (patent literature 2) using a power supply modulator configured by parallel-connecting a voltage source using a linear amplifier and a current source using a switching amplifier, and a technique (patent literature 3) using a power supply modulator configured by series-connecting a voltage source having a plurality of voltage values (including an error) and a voltage source having an error correction function.